1. Field of the Invention
The present invention relates to an antenna used for mobile radio equipment to be mounted on a vehicle or the like.
2. Description of the Related Art
Recently, a linear mono-pole antenna or a folded mono-pole antenna is generally employed as an antenna for mobile radio equipment to be mounted on a vehicle.
Such a conventional antenna will be described in the following with reference to FIG. 11.
FIG. 11(a) is a side view of a conventional mono-pole antenna. The conventional mono-pole antenna comprises a flat-plate conductive ground plane 91 made from copper material or the like, a feeding point 92 positioned at the center of the ground plane 91, and an antenna element 93 made from linear copper material or the like. The antenna element 93, with one end connected to the feeding point 92 and the other end opened, is vertically extended by height h against the ground plane 91. The mono-pole antenna is configured in this way.
Also, FIG. 11(b) is a side view of a conventional folded mono-pole antenna. The conventional folded mono-pole antenna has an antenna element 103 made from linear copper material or the like which is folded in U-shape. The antenna element 103, with one end connected to the feeding point 92, is vertically extended by height h against the ground plane 91, and its upper part is folded in U-shape, while the other end is connected to the ground plane 91. The folded mono-pole antenna is configured in this way.
In any of the antennas configured as described above, when high-frequency current of operating frequency is supplied to the feeding point 92, the antenna elements 93, 103 are excited to perform signal transmitting operation. Also, in signal reception, the antenna elements 93, 103 are excited by the high-frequency electromagnetic field of operating frequency to perform signal receiving operation.
The mono-pole antenna is formed so that one end of the antenna element 93 is connected to the feeding point 92 and the other end thereof is opened. Therefore, current (i1) across a–b and image current (i1) equivalent to across a–b flow in same phase to the ground plane 91. The mono-pole antenna is excited in this way, and then electromagnetic waves are emitted into the air.
On the other hand, the folded mono-pole antenna is formed so that the antenna element 103 is folded in U-shape. Therefore, in addition to current (i1) across a–b and current (i3) across c–d, image current (i1, i3) equivalent to across a–b and across c–d flows in same phase to the ground plane 91. Since the folded mono-antenna is enhanced in excitation by using such a configuration, the band width of the antenna can be expanded.
As preceding technical document information regarding the invention of this application, for example, Japanese Laid-open Patent S62-122401 is well known.
However, as in a conventional mono-pole antenna and a folded mono-pole antenna described above, the antenna is generally operated in a ¼ wavelength mode. Therefore, the mechanical height h is required to be at least ¼ wavelength of operating frequency. For example, in the case of 900 MHz band used for cellular phones, the height required is at least 83 mm that is equivalent to ¼ wavelength thereof.
Accordingly, for the reduction in size of the antenna, if the mechanical height h of the antenna element is lowered by making the height shorter than ¼ wavelength of operating frequency, then the antenna impedance is decreased, and there arises a problem that it is difficult to adjust impedance matching.
Also, when the above conventional antenna is installed on a rear tray or dashboard in a vehicle, it is desirable to face the antenna elements 93, 103 upward in order to improve the electromagnetic wave emission efficiency of the antenna. However, if the antenna elements 93, 103 are faced upward, a large space is occupied by the antenna elements 93, 103 in the direction of height, and there arises a problem of causing inconvenience in use of the antenna in a vehicle.